In the present treatise I propose discussing the most important fundamental principles of the treatment of renal diseases. Since my first publications concerning the pathology of the kidneys (Albuminuria in Healthy Men, 1885) I have continually devoted labor and interest to the study of these questions from their theoretical side as well as in their practical and empirical aspects, believing that I should no longer withhold the presentation of my views, especially as they differ in many points from the usual statements of the textbooks and manuals. Clinical and experimental investigations of these questions have been repeatedly undertaken by me or my students and the results published. Still the circle of these researches is not yet closed, in part because the works newly begun delay the completion, and in part because new complications of the question have been constantly arising. I will undertake here the discussion especially of the two chief forms of renal disease, acute nephritis and the contracted kidney. The one called the parenchymatous or better, sub-chronic, form of nephritis, does not require to be so exhaustively considered. Its changeable picture, which in its clinical behavior at one time approaches the acute and at another time the chronic contracted kidney, is less adapted to an analysis of the important fundamental therapeutic indications.